A. Field of the Invention
The present invention relates to an apparatus for monitoring and/or adjusting a tensile force in a yarn, said apparatus comprising yarn deflecting-elements one of which is displaceably held by a retention means that is fitted with a device for monitoring and/or adjusting its motion and/or its position and/or the torque it applies and/or the retention force it exerts.
B. Related Art
It is known from the German patent document 2,535,209 A1 to displaceably hold a central deflecting element by a swivel arm supported in a plane connecting two further deflecting elements. The swivel arm is held by an electric motor drive in a preselected deflecting position. The required current magnitude is a signal representing yarn tension or yarn tensile force.
It is further known from the German patent document 2,553,859 A1 to mount two mutually oppositely displaceable deflecting elements between two stationary deflecting elements on a common, two-arm swivel lever which is supported in a plane linking the two stationary deflecting elements. An electric motor drive is associated with the two-arm lever. The required pivot force is determined by measuring the electric power and is representative of yarn tension or yarn tensile force.
It is further known from the British patent 2,125,072 A to load an electromagnetic drive holding a central deflecting element with a constant drive force. Arm excursion is detected by an optical device to provide a measure of yarn tensile force or yarn tension.
Also, U.S. Pat. No. 4,010,915 discloses a yarn brake having two stationary deflecting elements and two oppositely displaceable deflecting elements between them. This yarn brake is regulated in a way such that the yarn tension remains substantially constant in front of the yarn brake. A tension sensor precedes the yarn brake for that purpose.
U.S. Pat. No. 5,462,094 also discloses a yarn brake consisting of two stationary deflecting elements and one displaceable deflecting element configured between them in a weaving machine filling insertion system. The yarn brake is mounted between a prewinder and a device inserting the fillings into a shed. The central deflecting element is displaceable transversely relative to the direction of yarn motion. The braking magnitude depends on the excursion implemented by the central deflecting element and is measured and regulated by a tension sensor.
Moreover a yarn brake for a weaving machine insertion system is known from WO 00/44970, said brake being mounted between a prewinder and a main blow nozzle of an airjet weaving machine. The yarn brake is composed of two stationary deflecting elements and a central displaceably held deflecting element. The position of the displaceable deflecting element is program-controlled. For that purpose the position of the central deflecting element is detected and then compared with a program-selected nominal position. In the event of discrepancies between the instantaneous position and the desired nominal position, the power applied to an electric drive motor adjusting the central element is changed in such manner that any discrepancy between the actual value and the nominal one shall be substantially eliminated.
In all the above apparatus, the yarn tensile force in the yarn segment beyond the central deflecting element is larger by the amount of friction between the yarn and the deflecting element than the yarn tensile force in the segment preceding the central deflecting element. Accordingly the force with which the retention means holds or supports the central deflecting element depends on the coefficient of friction between the particular yarn material and the deflecting element. In most cases however this coefficient of friction will not be known, and therefore the above designs do not enable accurately determining the yarn tension or yarn tensile force. This applies in particular to high yarn speeds on the plausible assumption that the coefficient of friction between the thread and the deflecting element varies with yarn speed.